1. Field of the Invention
The present invention generally relates to thermal transfer type printers, and more particularly to a thermal transfer type printer capable of varying a printing density thereof based on printing data which select desirable printing density.
2. Prior Art
Conventionally, a thermal transfer type printer prints bar codes, characters and graphics on a printing paper by use of a transfer ribbon and a line thermal head in which a plurality of heating cells are disposed in one line (hereinafter, referred to as a thermal head). More specifically, thermal melting ink is painted on a surface of the transfer ribbon so that an ink layer is formed on the surface of the transfer ribbon, and this ink layer of the transfer ribbon is pressed against the printing paper. The thermal head is pressed against the backside of the transfer ribbon and heated so as to melt the thermal melting ink of the ink layer in response to a desirable pattern. Such melted ink is transferred on the printing paper. Thus, the desirable pattern is printed on the printing paper. Other than this thermal transfer type printer, a thermosensible type printer is also well known. In such thermosensible type printer, a printing pattern is directly given to a thermosensible paper so that the printing pattern is printed on the thermosensible paper.
In the above-mentioned printers, the printing density is generally set to a predetermined fixed density by use of a volume and a switch. In some thermal transfer type printers, a density control circuit is provided for maintaining a high printing quality. More specifically, the density control circuit controls the heating temperature of the thermal head based on the present temperature of the thermal head detected by a thermistor so that the printing density is maintained at the predetermined fixed density. In addition, this density control circuit provides a memory (e.g., ROM) therein, which is written by data concerning a current-on time (i.e., a period for supplying current to the thermal head). As shown in FIG. 1, these current-on times are estimated from current-on characteristics (shown as a curve for supplied energy) which determine a value of a current supplied to the thermal head. The respective data of the current-on times obtained from the above curve (shown in FIG. 1) will be shown in the following Table 1 (shown in the next page).
TABLE 1 ______________________________________ Temp. (Degree Value of Temp. Data Current-On Time Centigrade) from D/A Converter 9 (milli second) ______________________________________ 60 1 0.51 2 0.52 3 0.53 4 0.54 5 0.55 252 2.99 0 253 3.00 ______________________________________
Based on such data of current-on times, a period for supplying the current to the thermal head is determined. For example, in the case where a printing operation must be stopped immediately after a power switch of a printer is turned on accidentally, the current-on time is set relatively longer because an initial temperature of the thermal head is relatively low. When the initial temperature of the thermal head is set at 0 degree centigrade, it is apparent from FIG. 1 that the desirable current-on time is 3 ms (i.e., 3 millisecond). On the contrary, when the temperature of the thermal head rises to a sufficiently high temperature, the current-on time can be shorten. For example, when the temperature of the thermal head is at 60 degrees centigrade, the desirable current-on time is 0.5 ms. As described above, the density control circuit detects the temperature of the thermal head by the thermistor (which is provided within the thermal head) and determines the desirable current-on time where the printing density is controlled to become constant.
Meanwhile, the conventional thermal transfer type printers include a bar code printer and a color printer and the like. Recently, such bar code printer is used in several fields, such as the factory automation (FA) field, a distribution industry field and the like. In addition, such color printer is used in the office automation (OA) field and the computer aided design (CAD) field and the like. Due to demands of the above-mentioned fields, highly fine-grained printing and high printing quality are required for the printer.
However, the printing density is maintained constant in the conventional thermal transfer type printer, regardless of kinds of the printing density. Hence, the conventional printer suffers a problem in that it is impossible for an external control device (such as a computer etc.) to vary the printing density in accordance with character patterns. A variable density switch enables the printer to vary the printing density of all printed patterns. Even in a printer having such variable density switch, however, it is impossible to vary the printing density by every character data.
Next, description will be given with respect to a variable density control of the thermal transfer type bar code printer, for example. When a density control condition is adjusted so that vertical bar codes are printed in a desirable printing density, the printing density of horizontal bar codes becomes faint and a clearance gap is formed between adjacent dots. On the contrary, when the density control condition is adjusted so that the horizontal bar codes are printed in a desirable printing density, the printing density of the vertical bar codes becomes too deep and the ink printed on one vertical bar code is flown over to the adjacent vertical bar code so that the two adjacent vertical bar codes are connected together by such flown ink. This causes an error in reading data from the bar codes using a bar code reader.
Incidentally, the horizontal bar codes differ from the vertical bar codes by a reading direction of the bar code reader. More specifically, data of the horizontal bar codes can be read by the bar code reader in a horizontal direction, and data of the vertical bar codes can be read by the bar code reader in a vertical direction.
FIGS. 2A, 2B, 3A and 3B show printed dots of the thermal transfer type bar code printer. More specifically, FIGS. 2A and 2B show horizontal bar codes which are read by the bar code reader in the horizontal direction indicated by an arrow H, and FIGS. 3A and 3B show vertical bar codes which are read by the bar code reader in the vertical direction indicated by an arrow V.
Further more specifically, FIG. 2a designates the horizontal bar codes in the case where the current-on time of the head is set relatively short. As shown in FIG. FIG. 2A, the printing density is therefore faint and a distance A is formed between two adjacent dots. This horizontal bar code must be formed in a continuous line, however, the horizontal bar code is actually formed in a dotted line. On the contrary, in the case where the current-on time of the thermal head is set relatively long as shown in FIG. 2B in order to prevent the above phenomenon, sizes of dots become large and the two adjacent dots are connected together so that the horizontal bar code is formed in the continuous line.
On the other hand, FIG. 3A designates vertical bar codes in the case where the current-on time of the thermal head is set relatively long. As shown in FIG. 3A, the printing density of the vertical bar codes becomes deep so that two adjacent vertical bar codes are connected by overflown ink. This phenomenon is called "tailing" phenomenon. In order to prevent such tailing phenomenon from occurring, the current-on time of the thermal head must be set short enough so as to make the sizes of dots small as shown in FIG. 3B.
As described heretofore, it is difficult to print the horizontal and vertical bar codes together on the printing paper and it is also difficult to control the printing densities of both bar codes at a constant printing density in the conventional thermal transfer type printer. Such difficulty of the conventional thermal transfer type printer also occurs in the conventional thermal transfer type color printer, in which the printing density can not be varied in response to the contents of the print data.